The present invention pertains to control circuits and, more particularly, to voltage control circuitry for accurately controlling an output voltage in an uninterruptible power supply system regardless of input voltage and load variations.
In order to furnish background material for the subject matter of the present invention, reference may be made to U.S. Pat. No. 3,360,714 in which a voltage regulator device or circuit having open and closed loop compensating means is described. More particularly, the invention described in that patent involves a voltage regulator circuit in which the line or input voltage is selectively supplied to a pair of output terminals through an auto transformer in response to the controlled operation of a switching network associated with the auto transformer. Further, a control circuit therein is employed to selectively actuate the switching network so that either the average or root mean square value of the output voltage is maintained substantially constant.
Although the aforedescribed prior art voltage regulator circuit has its uses, it does not have the function of an uninterruptible power system; namely, that of supplying continuous power to a critical load during failure of the main power source.
In the case of systems that are designed to have an "uninterruptible power supply" or "UPS", certain basic components or elements are provided. One of these is a controlled rectifier/battery charger or similar means that is connected to the available AC utility lines and functions along with other components to charge a set of batteries so that in the event of power failure the batteries will constitute an emergency source. This is critically important in the case of computer installations or the like where information would be lost if power were to fail suddenly and no alternate source were available. Generally, the batteries referred to are held in a floating state by means of a controlled rectifier/battery charger; the batteries are connected or tied to an inverter which serves to generate an output AC voltage to be supplied to the load equipment such as the afore-noted computer or similar equipment.
A difficulty arises in connection with the aforenoted UPS systems when it is desired to control the output AC potential to as little as one-half percent variation in voltage or potential and to do so with an extremely fast response time.
Accordingly, it is a primary object of the present invention to provide control circuitry for an uninterruptible power supply and to do so with very fast response and extremely accurate voltage control.
Another object is to produce stability so as to avoid oscillations in the output of the unit or modulation of the AC output.
A further object is to provide control circuitry in which the voltage feedback means does not need to have a very high gain and for that reason is effective to achieve stability for the system.
Yet other objects are to compensate or correct for input voltage and load variations within a time interval corresponding to the period of one cycle of the applied input signal; and to achieve this with extremely low cost both with respect to construction and operation of the system.
Yet another object is to limit the inverter's output current to a predetermined upper limit.
In fulfillment of the above objects, a primary feature of the present invention resides in the provision of both a feed forward arrangement and a feed back arrangement in the control circuitry which operates upon the inverter means so as to produce rapid and accurate adjustment of its output voltage. More particularly, the feed forward arrangement includes DC input voltage control means, as well as AC output current control or compensating means; while the feed back arrangement includes both voltage and current feed back means within the control circuitry such that the voltage feedback per se can be provided with relatively low gain.
Other and further objects, advantages and features of the present invention will be understood by reference to the following specification in conjunction with the annexed drawing, wherein like parts have been given like numbers.